Historically wet process phosphoric acid plants have used the following generalized scheme of water recycle for both conservation and economic reasons: From a cooling pond, water is taken to a digester where phosphate rock is digested with phosphoric acid and sulfuric acid to yield a net make phosphoric acid and gypsum (i.e. calcium sulfate dihydrate). These digesters are cooled by evaporating a part of the water. The 30 percent phosphoric acid produced from the above digestion is then concentrated to approximately 54 percent phosphoric acid by vacuum evaporation. The condensates from the cooling and the off gases from the 30 percent acid manufacture and the condensate from the concentration evaporators all contain substantial amounts of fluorine (F) generally in the form of H.sub.2 SiF.sub.6. These condensates are pumped to a barometric seal tank from which they go to ponds for cooling. All liquid spillage and washings as a result of maintenance also go to the pond. As a result of spray carry over, spillage and washing, the pond accumulates a P.sub.2 O.sub.5 content of the order to 0.3-0.4 percent. The main rejection mechanism of the fluorine from the fluorapatite feed to the plant is via the product phosphoric acid. Some of the F is rejected by inclusion in the gypsum cake. The sum of fluorine rejected via the above two mechanisms, is less than the F feed to the plant. The imbalance then appears in the process water which slowly increases its F content. Eventually the F content over the process water gets high enough and a new mechanism of rejection comes into play. This rejection mechanism, highly objectionable from an ecological viewpoint, involves F loss to the atmosphere from the pond probably in the form of SiF.sub.4. At some point in the processing all three rejection mechanisms must come to some equilibrium. Since the volume of the pond water is so great and the area it must cover so large it is very difficult to calculate the amount of F evolution. However, it is known there is F evolution which can be highly objectionable. The process water should be treated to eliminate or reduce the fluorine content to a negligible level for pollution reasons.
In the prior art, methods have been proposed for fluorine control and recovery from aqueous acidic solutions in a variety of forms which are the outgrowth of a multiplicity of known defluorination techniques. Many of these methods involve the precipitation of insoluble fluosilicates from either the 30 percent or 54 percent phosphoric acid. The removal from the 54 percent acid does not affect the F content of the process water. Removal from the 30 percent acid provides a partial control method and could effect an eventual reduction of F in the process water. Other processes involve the isolation of H.sub.2 SiF.sub.6 from the evaporator condensates. These processes suffer the disadvantage of being costly and of producing a product of minimal value such as aqueous H.sub.2 SiF.sub.6 solution. Other processes have been proposed whereby the process water is neutralized with lime or limestone. These processes are costly but acceptable if the process water must be rejected from the recycle system, for example, disposal to water sheds. The neutralization processes involve a change of pH from the &lt;2 range into the 3+ range. The effect of this change is twofold, (1) the F values are precipitated and (2) the phosphoric acid content is also neutralized and frequently precipitated as calcium phosphate. The latter effect causes a loss of phosphorus values. Neutralization of the acid values represented by fluorine acids and phosphoric acid with lime and/or carbonates leads to the formation of water from the associated H+ ions of the phosphoric and fluosilicic acids. Neutralization of the process water with for example, lime or limestone can be considered an acceptable means of treatment provided the process water is to be discarded. However, if such water is to be preserved for reuse, the loss of both the phosphorous values and acid values becomes economically unacceptable.